We propose the signal strength gradient (SSG) orientation constraints for simultaneous localization and mapping (SLAM) using Wi-Fi received signal strength (RSS) measurements. We show that under certain circumstances, the relative orientation between nearby trajectory segments can be recovered from the cosine similarity between their SSGs. We then show how to obtain trajectory segments and self-consistent SSGs by jointly segmenting Wi-Fi measurements and odometry. Because SSG orientation constraints inevitably contain outliers, we also evaluate the effectiveness of robust SLAM backends on the proposed constraints. Experiments show that Wi-Fi SLAM using the proposed method can correctly estimate orientations given topologically incorrect initialization on trajectories with little to no overlapping sections.